1. Field of the Invention
The invention relates to a catadioptric projection objective for imaging a pattern arranged in an object surface onto an image surface.
2. Description of the Related Art
Projection objectives of that type are employed on projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order create even finer structures, it is sought to both increase the image-end numerical aperture (NA) of the projection objective to be involved and employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm.
However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbé numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations).
In lithography, a flat (planar) image is essential to expose planar substrates, such as semiconductor wafers. However, generally the image surface of an optical system is curved, and the degree of curvature is determined by the Petzval sum. The correction of the Petzval sum is becoming more important in view of the increasing demands to project large object fields on planar surfaces with increased resolution.
One approach for obtaining a flat image surface and good color-correction is the use of catadioptric systems, which combine both refracting elements, such as lenses, and reflecting elements, such as mirror, preferably including at least one concave mirror. While the contributions of positive-powered and negative-powered lenses in an optical system to overall power, surface curvature and chromatic aberrations are opposite to each other, a concave mirror has positive power like a positive-powered lens, but the opposite effect on surface curvature without contributing to chromatic aberrations.
Further, the high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems. Measures that will allow reducing the number and sizes of lenses and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
Catadioptric projection objectives having at least two concave mirrors have been proposed to provide systems with good color correction and moderate lens mass requirements. The U.S. Pat. No. 6,600,608 B1 discloses a catadioptric projection objective having a first, purely refractive objective part for imaging a pattern arranged in the object plane of the projection objective into a first intermediate image, a second objective part for imaging the first intermediate image into a second intermediate image and a third objective part for imaging the second intermediate image directly, that is without a further intermediate image, onto the image plane. The second objective part is a catadioptric objective part having a first concave mirror with a central bore and a second concave mirror with a central bore, the concave mirrors having the mirror faces facing each other and defining an intermirror space or catadioptric cavity in between. The first intermediate image is formed within the central bore of the concave mirror next to the object plane, whereas the second intermediate image is formed within the central bore of the concave mirror next to the object plane. The objective has axial symmetry and provides good color correction axially and laterally. However, since the reflecting surfaces of the concave mirrors are interrupted at the bores, the pupil of the system is obscured.
The Patent EP 1 069 448 B1 discloses another catadioptric projection objective having two concave mirrors facing each other. The concave mirrors are part of a first catadioptric objective part imaging the object onto an intermediate image positioned adjacent to a concave mirror. This is the only intermediate image, which is imaged to the image plane by a second, purely refractive objective part. The object as well as the image of the catadioptric imaging system are positioned outside the intermirror space defined by the mirrors facing each other. Similar systems having two concave mirrors, a common straight optical axis and one intermediate image formed by a catadioptric imaging system and positioned besides one of the concave mirrors are disclosed in Japanese patent application JP 2002208551 A and US patent application US 2002/00241 A1.
European patent application EP 1 336 887 (corresponding to US 2004/0130806 A1) discloses catadioptric projection objectives having one common straight optical axis and, in that sequence, a first catadioptric objective part for creating a first intermediate image, a second catadioptric objective part for creating a second intermediate image from the first intermediate image, and a refractive third objective part forming the image from the second intermediate image. Each catadioptric system has two concave mirrors facing each other. The intermediate images lie outside the intermirror spaces defined by the concave mirrors. Concave mirrors are positioned optically near to pupil surfaces closer to pupil surfaces than to the intermediate images of the projection objectives.
International Patent application WO 2004/107011 A1 discloses catadioptric projection objectives having one common straight optical axis and two or more intermediate images and suitable for immersion lithography with numerical apertures up to NA=1.2. At least one concave mirror is positioned optically near to a pupil surface closer to that pupil surface than to an intermediate images of the projection objective.
In the article “Nikon Projection Lens Update” by T. Matsuyama, T. Ishiyama and Y. Ohmura, presented by B. W. Smith in: Optical Microlithography XVII, Proc. of SPIE 5377.65 (2004) a design example of a catadioptric projection lens is shown, which is a combination of a conventional dioptric DUV system and a 6-mirror EUV catoptric system inserted between lens groups of the DUV system. A first intermediate image is formed behind the third mirror of the catoptric (purely reflective) group upstream of a convex mirror. The second intermediate image is formed by a purely reflective (catoptric) second objective part. The third objective part is purely refractive featuring negative refractive power at a waist of minimum beam diameter within the third objective part for Petzval sum correction.
Japanese patent application JP 2003114387 A and international patent application WO 01/55767 A disclose catadioptric projection objectives having one common straight optical axis, a first catadioptric objective part for forming an intermediate image and a second catadioptric objective part for imaging the intermediate image onto the image plane of this system. Concave and convex mirrors are used in combination.
The article “Camera view finder using tilted concave mirror erecting elements” by D. DeJager, SPIE. Vol. 237 (1980) p. 292-298 discloses camera view finders comprising two concave mirrors as elements of a 1:1 telescopic erecting relay system. The system is designed to image an object at infinity into a real image, which is erect and can be viewed through an eyepiece. Separate optical axes of refractive system parts upstream and downstream of the catoptric relay system are parallel offset to each other. In order to build a system having concave mirrors facing each other mirrors must be tilted. The authors conclude that physically realizable systems of this type have poor image quality.
International patent applications WO 92/05462 and WO 94/06047 and the article “Innovative Wide-Field Binocular Design” in OSA/SPIE Proceedings (1994) pages 389ff disclose catadioptric optical systems especially for binoculars and other viewing instruments designed as inline system having a single, unfolded optical axis. Some embodiments have a first concave mirror having an object side mirror surface arranged on one side of the optical axis and a second concave mirror having a mirror surface facing the first mirror and arranged on the opposite side of the optical axis such that the surface curvatures of the concave mirrors define and intermirror space. A front refractive group forms a first intermediate image near the first mirror and a second intermediate image is formed outside of the space formed by the two facing mirrors. A narrow field being larger in a horizontal direction than in a vertical direction is arranged offset to the optical axis. The object side refractive group has a collimated input and the image side refractive group has a collimated output and entrance and exit pupils far from telecentric are formed. The pupil shape is semi-circular unlike pupil surfaces in lithographic projection lenses, which have to be circular and centered on the optical axis.
The PCT application WO 01/044682 A1 discloses catadioptric UV imaging systems for wafer inspection having one concave mirror designed as Mangin mirror.
Catadioptric projection objectives consisting of a catadioptric imaging subsystem having one single concave mirror and arranged between an entry side and an exit side refractive imaging subsystem (so-calles R-C-R systems) are disclosed, for example, in U.S. application with Ser. No. 60/573,533 filed on May 17, 2004 by the applicant. Other examples of R-C-R-systems are shown in US 2003/0011755, WO 03/036361 or US 2003/0197946.
US patent application with title “Catadioptric Projection Objective” filed by the applicant on Jan. 14, 2005 (based on U.S. provisional applications 60/536,248 filed on Jan. 14, 2004; U.S. 60/587,504 filed Jul. 14, 2004; 60/617,674 filed Oct. 13, 2004; 60/591,775 filed Jul. 27, 2004; and 60/612,823 filed Sep. 24, 2004) discloses catadioptric projection objectives having very high NA and suitable for immersion lithography at NA>1 with maximum values NA=1.2. The projection objectives comprise: a first objective part for imaging the pattern provided in the object plane into a first intermediate image, a second objective part for imaging the first intermediate imaging into a second intermediate image, and a third objective part for imaging the second intermediate imaging directly onto the image plane. The second objective part includes a first concave mirror having a first continuous mirror surface and a second concave mirror having a second continuous mirror surface, the concave mirror faces facing each other and defining an intermirror space. All concave mirrors are positioned optically remote from pupil surfaces. The system has potential for very high numerical apertures at moderate lens mass consumption. The full disclosure of this document and the priority documents thereof is incorporated into the present application by reference.